Low density polyethylene (LDPE) resins for extrusion coating and extrusion lamination on paper, board, aluminum, etc., are designed with broad MWD (molecular weight distribution) and low extractables. In extrusion coating applications, the following product and application properties are, among others, of importance: coating performance at variable processing speeds, adhesion to the substrate, barrier properties, and seal formation. The coating performance at variable processing speeds will depend mainly on the viscoelastic properties of the polymer, while adhesion, barrier, and sealing properties will also depend, in addition to the viscoelastic properties, on the density, crystallinity, and functionality of the polymer.
Typically LDPE resins with broad MWD are made using autoclave reactors or a combination of autoclave and tube reactors. Broad MWD resins can be achieved in autoclave reactors by promoting long chain branching, and through the inherent residence time distribution, by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths.
International Application No. PCT/U.S. Ser. No. 12/064,284, filed Nov. 9, 2012, describes LDPE resins made using tubular reactors that are suitable for extrusion coating. However, these resins are designed for the lower density end of the application space for extrusion coating (e.g. less than 0.920 g/cc). For the higher and/or medium density segment of extrusion coating applications, autoclave-based LDPEs are still required. Alternatively, blends of high melt strength LDPE of lower density with LLDPE or HDPE can be used to achieve higher overall density.
There is a need for compositions that comprise high-pressure ethylene-based polymers, such as LDPE polymers, to achieve a good balance of adhesion and substrate-polymer delamination. The presence of other polymer types, such as linear-type polymers, could compromise this balance, due to presence of anti-oxidants, and/or due to resulting mechanical properties that will promote undesired delamination.
There is also a need for flexibility in LDPE component selection to achieve higher density and good coating performance, without having to select exclusively an autoclave-based resin, and there is a need to achieve even higher overall density, e.g., for improved barrier properties, at good coating properties, that is also typically not reachable by only using a single autoclave-based resin (>0.926 g/cc). Standard autoclave (AC) coating grades in the “0.920-0.924 g/cc” density range do exist commercially. There is a need for new compositions that can be used to replace conventional compositions containing these “lower conversion AC resins,” which are costly to produce.
U.S. Publication 2007/0225445 describes blends of one AC with one tubular LDPE product. Both types of products have a broad MWD, high melt strength, and therefore a lower density. Data at different blend ratios show overall density to be low. Furthermore the melt index range of the tubular and the autoclave products should comply with, respectively, 4 to 10 g/10 min and 3 to 9 g/10 min ranges. These melt index ranges, in combination with blend composition ranges from 75/25 to 25/75, do not allow the use of a high density resin with a low melt elasticity, while maintaining in the final blend high melt elasticity to ensure good coating performance.
International Publication WO 2011/075465 describes the manufacturing and application of autoclave based resins, which have higher densities, and can be extrusion coated; however neck-in performance can be continued to be improved upon. The present invention results in even higher density capability at superior extrusion coating processing performance, with no compromise to neck-in performance.
EP2123707B1 describes a blend of majority of tubular with 2-30 wt % of autoclave of MI>2.5, with certain viscosity relations. The use of a higher melt index, high melt strength blend resin results in blends with lower melt strength extrusion coating performance behavior and reduces maximum density capability.
There remains a need for new ethylene-based polymer compositions comprising an optimum balance of processing, density, crystallinity, and end-use performance in extrusion coatings. This need and others have been met by the following invention.